1. Technical Field
The present invention relates to the Global Positioning System (GPS) in general, and in particular to GPS receivers. Still more particularly, the present invention relates to an apparatus for providing anti-jamming capability to legacy GPS receivers.
2. Description of Related Art
The Global Positioning System (GPS) was originally designed and developed to serve military needs. However, over the years, GPS applications have been extended from the military arena to the commercial arena. GPS applications in the commercial arena include automobile navigation, automatic vehicle location, land surveying, aircraft navigation and landing systems, precise timing systems, etc.
GPS radio navigation relies upon a constellation of twenty-four active (plus active spare) GPS satellites travelling in six different orbital planes around the earth. GPS position fixes are obtained by measuring the propagation delay times of radio signals broadcasted by the orbiting GPS satellites. Typically, a user must receive GPS radio signals from at least four satellites in order to solve for three position variables as well as a timing error that are needed to precisely determine a location and time.
Generally speaking, GPS radio signals are approximately 30 dB below the thermal noise floor. In other words, the level of GPS radio signals received on earth is only 1/1000th as strong as the noise environment around a GPS receiver attempting to track the GPS radio signals. The challenge of tracking GPS radio signals is further compounded by the presence of interference or jamming. Sophisticated techniques have been developed to combat jamming, but the implementations of those techniques are not always practical for legacy GPS receivers.
For example, current digital anti-jamming systems, which provide GPS anti-jamming protection by simultaneously pointing beams at GPS satellites of interest and nulling interference sources, are typically implemented with a digital GPS receiver and an anti-jamming electronic unit. However, many legacy aircraft navigation systems are not able to take advantage of the above-mentioned beam-steering technique without their analog GPS receiver being replaced. The replacement of a GPS receiver typically requires software to be updated and aircraft wiring to be replaced. In addition, the new navigation system must be re-qualified or re-certified. Thus, there is a significant expense involved in applying state-of-the-art anti-jamming technology to a legacy aircraft navigation system.
Consequently, it would be desirable to provide an improved apparatus for equipping legacy aircraft navigation systems with state-of-the-art anti-jamming capability.